Recently, a rechargeable secondary battery has been widely used as an energy source of a wireless mobile device.
In addition, the secondary battery has been interested as a power source of an electric vehicle (EV), a hybrid electric vehicle (HEV), and the like, that have been suggested as a scheme for solving air pollution of the existing gasoline vehicle, diesel vehicle, or the like, using a fossil fuel.
Small-sized mobile devices use one or several battery cells per one device. In contrast, medium or large-sized devices such as a vehicle, or the like, use a medium and large-sized battery pack in which a plurality of battery cells are electrically interconnected as a unit cell, to meet the needs of high output and large capacity.
Since it is preferable that the medium or large-sized battery packs are manufactured to have a size and a weight as small as possible, they may be stacked with high integration. In addition, a prismatic battery, a pouch type battery, or the like, having a small weight against a capacity has been mainly used as a battery cell of the medium or large-sized battery pack. Among those, the pouch type battery having a small weight, less leakage possibility of an electrolyte, and low manufacturing costs has drawn much attention.
As unit batteries (battery cells) of the medium or large-sized battery pack, nickel-hydrogen secondary batteries have been mainly used. However, recently, many studies on lithium secondary batteries providing high output against capacity like the small battery pack, have been conducted, and some of the lithium secondary batteries have been commercialized.
However, the lithium secondary battery basically has low stability. The lithium secondary battery has an electrode assembly which is accommodated in a case into which an electrolyte is injected. In this case, when the electrode assembly is overcharged to increase a current and a voltage, the electrolyte inside the case is decomposed due to overheating to generate inflammable gas, and as a result, a swelling phenomenon that the case itself is swollen occurs. In this case, heat is not diffused inside the case as a whole and excessively diffused at a specific position of the case, and thus a separation membrane is melted between a cathode and an anode. As a result, the cathode and the anode are short-circuited and thus ignition occurs.
As described above, there is a need to develop an electrode assembly of a secondary battery for solving a problem of an internal short circuit which occurs due to an internal deformation under the overcharging condition.